# MEASUREMENT OF FUNCTIONAL RESIDUAL CAPACITY AND RESIDUAL VOLUME

MEASUREMENT OF FUNCTIONAL RESIDUAL CAPACITY AND RESIDUAL VOLUME

Residual volume and the functional residual capacity cannot be measured by spirometer and can be determined by three methods:

1. Helium dilution technique

2. Nitrogen washout method

3. Plethysmography.

HELIUM DILUTION TECHNIQUE

Procedure to Measure Functional Residual Capacity

Respirometer is filled with air containing a known quantity of helium. Initially, the subject breathes

normally. Then, after the end of expiration, subject breathes from respirometer. Helium from respirometer

enters the lungs and starts mixing with air in lungs. After few minutes of breathing, concentration of helium in the respirometer becomes equal to concentration of helium in the lungs of subject. It is called the equilibration of helium. After equilibration of helium between respirometer and lungs, concentration of

helium in respirometer is determined. Functional residual capacity is calculated by the

formula:

V (C1 – C2)

FRC =

C2

Where,

C1 = Initial concentration of helium in the

respirometer

C2 = Final concentration of helium in the

respirometer

V = Initial volume of air in the respirometer.

Measured Values

For example, the following data of a subject are obtained from the experiment:

1. Initial volume of air in respirometer = 5 L (5,000 mL)

2. Initial concentration of helium in respirometer = 15%

3. Final concentration of helium in respirometer = 10%.

Calculation

From the above data, the functional residual capacity of the subject is calculated in the following way:

V (C1 – C2)

FRC =

C2

5,000 (15/100 – 10/100)

FRC = mL

10/100

5,000 (5/100)

= mL

10/100

5,000 × 5

= mL

10

= 2,500 mL

Thus, the functional residual capacity in this subject is 2,500 mL.

Procedure to Measure Residual Volume

To determine functional residual capacity, the subject starts breathing with respirometer after the end of

normal expiration. To measure residual volume, the subject should start breathing from the respirometer

after forced expiration.

NITROGEN WASHOUT METHOD

Normally, concentration of nitrogen in air is 80%. So, if total quantity of nitrogen in the lungs is measured, the volume of air present in lungs can be calculated.

Procedure to Measure Functional

Residual Capacity

Subject is asked to breathe normally. At the end of normal expiration, the subject inspires pure oxygen through a valve and expires into a Douglas bag. This procedure is repeated for 6 to 7 minutes, until the nitrogen in lungs is displaced by oxygen. Nitrogen comes to the Douglas bag. Afterwards, following factors are measured to calculate functional residual capacity.

Calculation

i. Volume of air collected in Douglas bag

ii. Concentration of nitrogen in Douglas bag.

By using the data, the functional residual capacity is calculated by using the formula:

C1 × V

FRC =

C2

Where,

V = Volume of air collected

C1 = Concentration of nitrogen in the collected

air

C2 = Normal concentration of nitrogen in the

air.

Measured Values

For example, the following data are obtained from the experiment with a subject:

i. Volume of air collected = 40 L (40,000 mL)

ii. Concentration of nitrogen = 5%

in the collected air

iii. Normal concentration of = 80% nitrogen in the air.

Calculation

From the above data, the functional residual capacity of the subject is calculated in the following way:

C1 × V

FRC =

C2

5/100 × 40,000

FRC = mL

80/100

5 × 40,000

= mL

80

= 2,500 mL.

Thus, functional residual capacity in this subject is 2,500 mL.

Procedure to Measure Residual Volume

To measure the functional residual capacity, the subject starts inhaling pure oxygen after the end of normal expiration and to determine the residual volume, the subject starts breathing pure oxygen after forceful expiration.

PLETHYSMOGRAPHY

Plethysmography is a technique to study the variations in the size or volume of a part of the body such as

limb. Plethysmograph is the instrument used for this purpose. Whole body plethysmograph is the instrument used to measure the lung volumes including residual volume.

Plethysmography is based on Boyle’s law of gas, which states that the volume of a sample of gas is

inversely proportional to the pressure of that gas at constant temperature.

Subject sits in an airtight chamber of the whole body plethysmograph and breathes normally through

a mouthpiece connected to a flow transducer called pneumotachograph.